The present invention relates to a stationary induction apparatus of the type having a laminated core composed of a plurality of sheets stacked in layers each being constituted by a plurality of members abutting and jointed to each other at the corners of the core in such a manner that two members of alternating layers partially overlap each other over a predetermined region at each corner of the core.
FIG. 4 shows a portion of the core of an ordinary stationary induction apparatus which is typically but not exclusively a single-phase transformer, particularly a corner of the core where a plurality of members constituting the core are jointed in an abutting relation. More specifically, the core has a laminated structure composed of a multiplicity of layers each of which is constituted by a plurality of members. For instance, each layer of the core has a yoke member 1 and a leg member 2 which abut each other at a corner of the core, in such a manner that yoke member 1 and the leg member 2 of alternating layers partially overlap each other over a hatched area (referred to as overlapping area 3, hereinafter) in FIG. 4, as will be described hereinunder.
FIGS. 5 and 6 show two adjacent layers of the core at the corner portion shown in FIG. 4. The yoke member 1A and the leg member 2A of a first layer shown in FIG. 5 are rolled iron sheet members which are obliquely cut at 45.degree. with respect to the rolling direction shown by arrows A so as to make full use of the advantage of the rolled sheet, and joined to each other at their obliquely cut ends. Similarly, the yoke member 1B and the leg member 2B of a second layer shown in FIG. 6 are rolled iron sheet members which are obliquely cut at 45.degree. with respect to the rolling direction shown by arrows A and joined to each other at their obliquely cut ends. It will be seen, however, the positions where the yoke member 1A and the leg member 2A of the first layer are cut are altered from the positions where the yoke member 1B and the leg ember 2B are cut, as will be understood from FIGS. 5 and 6. Thus, in the first layer shown in FIG. 5, a triangular notch 4 is formed by one longitudinal side of the leg member 2A and the oblique cut edge of the yoke member 1A, whereas, in the layer shown in FIG. 6, a triangular notch 4 is formed between one longitudinal side of the yoke member 1B and the oblique cut edge of the leg member 2B.
It will be seen that, when a plurality of first layers as shown in FIG. 5 and a plurality of second layers as shown in FIG. 6 are prepared and both types of layers are stacked alternatingly, the yoke members and the leg members of the alternating layers are partially overlapped in the overlapping region 3 shown by hatching in FIG. 4. In other words, in the overlapping region 3, the yoke member and the leg member are stacked alternatingly. It will also be seen that the total thickness of the iron layer in each of the triangular notched regions 4, 4 in FIG. 4 is half that in other regions because in such a region the iron layer and vacancy appears alternatingly in the direction of thickness of the core.
Representing the width of the frame by W and the length of the shorter sides of isosceles triangle forming the notch 4 by E, the area S of the overlapping region 3 is given by the following formula (1). EQU S=E(2W-E) (1)
In the event of an accident such as a short-circuiting in the induction apparatus, an abnormal physical force is applied to the core. This physical force is borne by the friction between the adjacent layers in the overlapping region 3.
This frictional force F is given as follows, representing the pressure at which the layers of the core are tightened by P and the friction coefficient between the adjacent layers in the overlapping region 3 by .mu.. EQU F=.mu..PS (2)
The formula (1) above suggests that, in order to attain a high frictional force F for the purpose of sustaining a large abnormal force, it is necessary that the overlapping region area S be increased where the friction coefficient .mu. and the tightening pressure P are constant.
Thus, the known core structure explained above suffer from the following problem.
It is to be pointed out that the magnetic flux in the core is undesirably disturbed by the presence of the notched portions 4,4 where the total thickness of the iron is smaller than in other regions, with the result that the iron loss or hysteresis loss of the induction apparatus is increased.
It is also to be pointed out that a greater mechanical strength of the core requires a greater frictional force and, hence, a greater area S of the overlapping region 3. This, however, essentially requires that the length E in formula (1) mentioned above be increased. In consequence, the size of the notched portion 4 is increased resulting in a greater iron loss.
From the formula (2), it will be understood that the frictional force F and, hence, the mechanical strength of the core would be increased by increasing the tightening pressure P. The higher tightening pressure, however, essentially requires that the strength of the tightening structure be increased correspondingly. This is inconvenient from an economical point of view.